Neolignans and lignans are known for their wide range of biological activities including hapatoprotective, hormone blocking, antibacterial, antifungal, plant growth regulator, anti-HIV, anticancer and antioxidant activities (Macrae, W. D. and Towers, G. H. N., Phytochemistry, 23 (6), 1207-1220 (1984); Ward, R. S., Tetrahedron, 46 (15), 5029-5041 (1990); Chariton, J. L., J. Nat. Prod., 61, 1447-1451 (1998); Alves, C. N.; Barroso, L. P.; Santos, L. S. and Jardim, I. N, J. Braz. Chem. Soc., 9(6), 577-582 (1998); Juhasz, L.; Dinya, Z.; Antus, S. and Gunda, T. E., Tetrahedron Letters, 41, 2491-2494 (2000); Tanaka, T.; Konno, Y.; Kuraishi, Y.; Kimura, I.; Suzuki, T. and Kiniwa, M., Biorg. & Med. Chem. Letts., 12, 623-627 (2002); U.S. Pat. Nos. 6,294,574; 6,201,016; 5.856,323; 5,639,782; 5,530,141; 4,704,462; 4,619,943 and 4,540,709; JP Patent No. 4082837; WO Patent No. 09215294 and EP Patent No. 159565)). Neolignans and lignans are a large group of natural products characterized by the coupling of two C6-C3 units which axe derived from cinnamic acid derivatives, however, both are present in traces in plants (Rao, K. V. and Rao, N. S. P., J. Nat. Prod., 53 (1), 212-215 (1990) and Filler, F.; Bail, J. C. L.; Duroux, J .L.; Simon, A. and Chulia, A. J., Planta Medica, 67, 700-704 (2001)). For nomenclature purposes, the C6-C6 unit is treated as propylbenzene and numbered from 1 to 6 in the benzene ring from 7 to 9 (or α to γ) starting from propyl group. With the second C6-C6 unit the numbers are primed. When the two C6-C6 units are linked by a bond between positions 8 and 8′ (or β and β′), the compound is referred as a lignan. In the absence of the C-8 to C-8′ (or β and β′) bond, and where the two C6-C3 units are linked by a carbon-carbon bond, compound is referred to as neolignan. Dimers with linkages other than this type are known as cycloneolignan, epoxyneolignan and oxyneolignan etc. Similarly, the presence of a double bond (or triple bond) in the side chain (i.e. C-7 to C-9 or C-7 to C-9) of the lignan, neolignan or epoxyneolignan skeleton is indicated by changing the -ane ending to -ene (or -yne) with a locant to indicate the position of the double bond (Moss, G. P. Pure Appl. Chem., 72(8), 1493-1523 (2000)). The basic ring system of these neolignans and lignans can be deduced by dimerization of alilyl and p-propenylphenols (such as isoeugenol, coniferyl or sinapyl alcohol). Oxidation of phenols often yields phenoxy radicals, which couple with little selectivity. Both C—C and C—O bonds are formed, mainly in ortho- and para- positions to the phenolic hydroxyl. Synthetically useful reactions are obtained only when the reactivity is blocked by substituents in the aforementioned positions. For instance from 2,6- or 2,4-substituted phenols, C—C bonded biphenyls can be obtained in good yields. In other cases coupling can be directed by carrying out the reaction intramolecularly, ring closure being an effective way of inducing regioselectivity (Whitting, D. A. Oxidative Coupling of Phenols and Phenol Ethers. In Comprehensive Organic Synthesis, Trost, B. M.; Fleming, I.; Pattenden, G., Eds.; Pergemon: Oxford, Vol. 3, 659-703 (1991)). Similarly, oxidation of a mixture of two phenols can lead to a mixture of dimers of the individual phenols and cross-coupling products between the different phenols. When one phenol reacts much faster than the other, for instance if it has a lower oxidation potential, it tends to dimerize without formation of significant amounts of cross-coupling products (Syrjanen, K. and Brunow, G., J. Chem. Soc. Perkin Trans 1, 3425-3429 (1998)). One approach to this problem is to start with the less reactive phenol in large excess, and continuously add the more reactive phenol (and the oxidant) at a rate which is slow enough to keep its concentration too low for significant dimerisation. But this method is cumbersome and leads to a large reaction volumes, and is also difficult to reproduce. A wide range of oxidants such as K3Fe(CN)6, H2O2, FeCl3, VOF3, thallium (III) tristrifluoacetate, horseradish peroxidase, iodobenzene diacetate (Frank, B. and Schlingloff, G., Liebig. Ann. Chem., 659, 132 (1962); Taylor, W. I. and Battersby, A. R. In “Oxidative Couplings of Phenols”, Marcel Dekker, N.Y. (1967); Kametani, T. and Fukumoto, K., Synthesis, 657 (1972); Taylor, E. C.; Andrade, J. G.; Rall, G. J. H. and McKillop, A., J. Am. Chem. Soc., 93, 4841 (1971); Kaisa, S. and Gösta, B., Tetrahedron, 57,365-370 (2001); Juhaasz, L.; Kfürti, L. and Antus, S., J. Nat. Prod, 63, 866-870 (2000)) and many others have been used for oxidative coupling but generally these reagents gave poor yield, and often complex mixtures. Indeed, phenoxy radical or phenoxonium ion intermediate is most common for synthesis of lignans and neolignans but there are a few patents and papers where non-phenolic compounds have been used for the synthesis of lignans and neolignans (Kadota, S.; Tsubono, K. and Makino, K., Tetrahedron Letters, 28 (25), 2857-2860 (1987) and Dhal, R.; Landais, Y.; Lebrun, A.; Lenain, V. and Robin, J. P., Tetrahedron, 50(4), 1153-1164 (1994)). For example, nordihydroguaiaretic acid (one of the most important dimer derived from resinous exudates of many plants), associated with a wide range of pharmacological activities, including the inhibition of the human papillomavirus, herpes simplex, HIV and hyperglycemic activity, has been synthesized by dimerization of non-phenolic compounds such as dimethoxypropiophenone (Perry, C. W. U.S. Pat. No. 3,769,350 (1975)), substituted benzylmagnesium chloride (Akio, M.; Kohei, T.; Keizo, S. and Makoto, K. Tetrahedron Letters, 21,4017-4020 (1980)) and dimethoxyphenylacetone (Mikail, H. G. and Barbara, N. T. Tetrahedron Letters, 42, 6083-6085 (2001)). However, above methods have a number of disadvantages including special handling of reagents, maintaining temperature below zero degree, expensive reagents and overall low yield, hence, none of the synthetic methods can be scaled up for industrial exploitation. On the contrary, the present invention is free from above drawbacks and discloses one step dimerisation of 2,4,5-trimethoxyphenylpropane (a dihydro product of asarone obtained via hydrogenation of β-asarone rich Acorus calamus oil) of the formula I (Example I) into novel neoliguan 3-ethyl-2-methyl-3-(2″,4″,5″-trimethoxy)phenyl-1-(2′,4′,5′-trimethoxy)phenyl-1-propene (named as NEOLASA-I) of the formula II (Example II). Further, neolignan (NEOLASA-I) is hydrogenated to obtain its corresponding dihydro product (3-ethyl-2-methyl-3-(2″,4″,5″-trimethoxy)phenyl-1-(2′,4′,5′-trimethoxy) phenylpropane) (named as NEONLASA-II) (Example III) so as to confirm the structure as well as to determine the position of double bond existing in the above parent neolignan (NEOLASA-I) of the formula (II) which may additionally serve as a simple synthon towards preparation of naturally occurring rare neolignans (such as acoradin or magnosalin or heterotropan and phenyl indane derivative) and their analogues in sufficient quantity to have opportunity for a wide range of biological activities (Wenkert, E.; Gottlieb, H. E.; Gottlieb, O. R.; Pereira, M. O. D. S. and Formiga, M. D., Phytochemistry, 15, 1547-1551 (1976); Kikuchi, T.; Kadota, S.; Yanada, K.; Tanaka, K.; Watanabe, K.; Yoshozaki, M.; Yokoi, T. and Shingu, T., Chem. Pharm. Bull. 31, 1112 (1983); Yamamura, S.; Niwa, M.; Nonoyama, M. and Terada, Y. Tetrahedron Letters, 4891 (1978); Kadota, S.; Tsubono, K.; M0kino, K.; Takeshita, M. and Kikuchi, T., Tetrahedron Letters, 28 (25), 2857-2860 (1987); Shimomura, H.; Sashida, Y and Oohara, M., Phytochemistry, 26(5), 1513-1515 (1987); Ahn, B.T.; Lee, S.; Lee, S. B.; Lee, E. S.; Kim, J. G. and Jeong, T. S., J. Nat. Prod., 64, 1562-1564 (2001) and Filleur, F.; Le Bail, J. C.; Duroux, J. L.; Simon, A. and Chulia, A. J., Planta Medica, 67, 700-704 (2001)).
In fact, formation of neolignan was observed accidentally when we were interested to develop a simple and economical process for the preparation of α-asarone, a well known hypolipideamic and antiplatelet active phenylpropanoid (Hernandez, A.; Lopez, M. L.; Chamorro, G. and Mendoza, F. T., Planta Medica, 59 (2), 121-124 (1993); Garduno, L.; Salazar, M.; Salazar, S.; Morelos, M. E.; Labarrios, F.; Tamariz, J. and Chamorro, G. A., J. of Ethnopharmacology, 55 (2), 161-163, (1997) and (Janusz, P.; Bozena, L.; Alina, T. D.; Barbara, L.; Stanislaw, W.; Danuta, S.; Jacek, P.; Roman, K.; Jacek, C.; Malgorzata, S. and Zdzislaw, C., J. Med. Chem., 43, 3671-3676 (2000)), via treatment of 2,4,5-trimethoxyphenylpropane of the formula I with DDQ in acetic acid into 1-(2,4,5-trimethoxy)phenyl-1-acetoxypropane followed by alkaline hydrolysis and its acidic dehydration to obtain α-asarone. This concept was based upon the reported method wherein treatment of benzylic compound with Hg(OAC)2/AcOH or DDQ/AcOH provided corresponding acetate derivative (Rao, K. V. and Chattopadhyay, S. K., Tetrahedron, 43, 669 (1987) and Rao, K. V. and Rao, N. S. P., J. Nat. Prod. 53(1), 212-215 (1990)). But to our surprise, the treatment of 2,4,5-trimethoxyphenylpropane (benzylic compound) with DDQ (1.0-1.3 moles) in the presence of acetic acid, provides mixture of unexpected products namely neolignan (32% yield), α-asarone (9% yield) and 1-(2,4,5-trimethoxy)phenyl-1-propanone (22% yield) (Example II) without formation of expected 1-phenyl-1-aceoxypropane derivative (Subodh, K. J. Org. Chem. 50, 3070-3073 (1985) and Ward, R. S. Tetrahedron Letters, 48 (15), 5029-5041 (1990)). The structure of neolignan (3-ethyl-2-methyl-3-(2″,4″,5″-trimethoxy)phenyl-1-(2′,4′,5′-trimethoxy)phenyl-1-propene or 2,2′,4,4′,5-5′-hexamethoxy-7′,8-neolig-7-ene), α-asarone and 1-(2,4,5-trimethoxy)phenyl-1-propanone (or isoacoramone) are successfully confirmed on the basis of spectral data (Example II). The formation of all the three products are postulated only when a part of 2,4,5-trimethoxyphenylpropane (C6-C3) undergoes dehydrogenation with DDQ towards formation of α-asarone while little other part of 2,4,5-trimethoxyphenylpropane undergoes oxidation with DDQ for isoacoramone formation. However, neolignan formation is possible only if some part of initially formed α-asarone undergoes rearrangements with unreacted 2,4,5-trimethoxyphenylpropane and DDQ towards dimerisation. Further, detailed mechanistic studies for above products are in progress. It is worthwhile to mention that increase in the amount of DDQ (1.4-2.1 moles) in acetic acid gave once again neolignan (NEOLASA-I) and α-asarone but 1-(2,4,5-trimethoxyphenyl)-1-propanone in little higher yield (39%) than above (22%) (Example II). Later on, like α-asarone, isoacoramone (2,4,5-trimethoxypropiophenone) is also realized as an interesting rare phenylpropanoid occurring in well known medicinal plants Acorus calamus, Piper marginatum as well as in Acorus tararinowii but only in traces (Mazza, G., J. of Chromatography, 328, 179-206 (1985); Santos, B. V. de O. and Chaves, M. C. de O., Biochem. Systematics Ecology, 25, 539-541 (1999) and Jinfeng, Hu and Xiaozhang, Feng, Planta Medica, 66, 662-664 (2000).
In conclusion, our invention discloses a simple and economical process for preparing novel neolignans (3-ethyl-2-methyl-3-(2″,4″,5″-trimethoxy)phenyl-1-(2′,4′,5′-trimethoxy)phenyl-1-propene of the formula (II) and 3-ethyl-2-methyl-3-(2″,4″,5″-trimethoxy)phenyl-1-(2′,4′,5′-trimethoxy)phenylpropane of the formula (III) along with α-asarone of formula (IIa), and isoacaromone (2,4,5-trimethoxypropiophenone) of formula (IIb), as side products thereof, starting from relatively cheaper and economical material 2,4,5-trimethoxyphenylpropane obtained via hydrogenation of β-asarone rich Acorus calamus oil as outlined in Scheme-I. Other objectives and advantages of the present invention will be made apparent as the description progresses. 